The Illinois State Water Survey (ISWS) and the U.S. Geological Survey (USGS) completed drilling of a new deep sandstone monitoring well in December 2015. The monitoring well is located in Kendall County, southeast of the town of Newark, IL. The 1,180 feet deep borehole has two nested wells, one which is open to the St. Peter Sandstone and one that is open to the deeper Ironton-Galesville Sandstone. The wells will monitor changing groundwater levels in these aquifers, which are an important source of groundwater for municipal and industrial needs.

Water levels in the St. Peter and Ironton-Galesville Sandstones have declined by over 800
feet since pre-development in parts of western Will and northern Kendall Counties. The network
of sandstone wells in these areas is dense and intricate and consists of wells that are open
to both sandstone aquifers, or wells that are only open to one sandstone aquifer. If wells
connect these two sandstone aquifers, then the difference between water levels (heads) in
each aquifer is small or nonexistent. In areas where there are no well connections between
the aquifers, there could be a difference in water levels of hundreds of feet.

Drilling the Well

Groundwater studies and groundwater flow modeling conducted by the ISWS require an understanding of the head separation between these two sandstone aquifers not only in the vicinity of groundwater pumping, but also in areas where there is little demand on groundwater. To this end, the drill site was selected in a sparsely populated area of Kendall County where demands from the sandstone aquifers are small. The site is also located south of the Sandwich Fault Zone, which is a deep bedrock fault that influences regional groundwater flow. The head difference between the St. Peter and Ironton-Galesville at this site provides insight into the regional groundwater flow across the fault zone to the large cone of depression that has propagated in western Will and northern Kendall Counties.

The borehole was drilled over the course of two weeks using mud rotary drilling. The wells were designed to ensure that the St. Peter and Ironton-Galesville Sandstone water levels could be obtained separately. Two PVC wells were installed in the same borehole, with grout separating the two aquifers. The wells are 2 inches in diameter and have 60 feet of screen open to each aquifer. The ISWS logged and collected drill cuttings during the drilling process. In addition to logging the cuttings, Tim Young at the Illinois State Geological Survey (ISGS) conducted geophysical logging of the borehole before well construction, measuring Gamma, Single Point Resistance (SPR), Resistivity, and borehole diameter.

After the wells were developed, water levels were obtained using an electric dropline. The depth to water for the St. Peter well was measured at 156.33 feet. The depth to water for the Ironton-Galesville well was measured at 225.92 feet. The 70 feet of head difference between the two aquifers is about half of what had previously been simulated by the ISWS using groundwater flow models. Recalibration of the northeastern Illinois model to better fit this current head separation is underway. Pressure transducers and a telemetry system were also installed in March 2016 to record water levels on a real-time continuous basis.

The Illinois State Water Survey at the
University of Illinois at Urbana-Champaign, a division of the Prairie Research
Institute, is the primary agency in Illinois concerned with water and
atmospheric resources.